Low molecular weight fibrinogen (LMW-F) isolated from Kabi fibrinogen will be compared with that purified from human plasma under conditions of a complete inhibition of proteolytic degradation. Subunit chain analysis, clottability and electrophoretic mobility of LMW-F will be compared with high molecular weight fibrinogen (HMW-F). Both fibrinogen fractions will be labelled with 14C and used for catabolic studies together with 14C labelled fibrinogen and fibrin degradation products. Distribution of radioactivity between intravascular and extravascular space and between various soluble and insoluble fractions of liver, lung, kidney and muscle tissues will be determined. Two forms of plasminogen activator isolated from the blood by an affinity procedure will be compared as to their ability to degrade plasma fibrinogen and fibrin.